Re: [patch 3/3] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device

[Peter Xu <peterx@redhat.com>]

On Tue, May 25, 2021 at 10:41:18AM -0300, Marcelo Tosatti wrote:
> For VMX, when a vcpu enters HLT emulation, pi_post_block will:
> 
> 1) Add vcpu to per-cpu list of blocked vcpus.
> 
> 2) Program the posted-interrupt descriptor "notification vector" 
> to POSTED_INTR_WAKEUP_VECTOR
> 
> With interrupt remapping, an interrupt will set the PIR bit for the 
> vector programmed for the device on the CPU, test-and-set the 
> ON bit on the posted interrupt descriptor, and if the ON bit is clear
> generate an interrupt for the notification vector.
> 
> This way, the target CPU wakes upon a device interrupt and wakes up
> the target vcpu.
> 
> Problem is that pi_post_block only programs the notification vector
> if kvm_arch_has_assigned_device() is true. Its possible for the
> following to happen:
> 
> 1) vcpu V HLTs on pcpu P, kvm_arch_has_assigned_device is false,
> notification vector is not programmed
> 2) device is assigned to VM
> 3) device interrupts vcpu V, sets ON bit
> (notification vector not programmed, so pcpu P remains in idle)
> 4) vcpu 0 IPIs vcpu V (in guest), but since pi descriptor ON bit is set,
> kvm_vcpu_kick is skipped
> 5) vcpu 0 busy spins on vcpu V's response for several seconds, until
> RCU watchdog NMIs all vCPUs.
> 
> To fix this, use the start_assignment kvm_x86_ops callback to kick
> vcpus out of the halt loop, so the notification vector is 
> properly reprogrammed to the wakeup vector.
> 
> Reported-by: Pei Zhang <pezhang@redhat.com>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> 
> Index: kvm/arch/x86/kvm/vmx/posted_intr.c
> ===================================================================
> --- kvm.orig/arch/x86/kvm/vmx/posted_intr.c
> +++ kvm/arch/x86/kvm/vmx/posted_intr.c
> @@ -236,6 +236,13 @@ bool pi_has_pending_interrupt(struct kvm
>  		(pi_test_sn(pi_desc) && !pi_is_pir_empty(pi_desc));
>  }
>  
> +void vmx_pi_start_assignment(struct kvm *kvm)
> +{
> +	if (!irq_remapping_cap(IRQ_POSTING_CAP))
> +		return;
> +
> +	kvm_make_all_cpus_request(kvm, KVM_REQ_UNBLOCK);

Shall we add a simple comment block explaining why we need this?

> +}

The patch itself looks right to me.

Reviewed-by: Peter Xu <peterx@redhat.com>

Thanks,

-- 
Peter Xu